I definitely wouldn't say that "most Americans would never pluralize 'cake.'" I'd say it's more that there are usage-level differences in when you'd use which.
The problem is that "cake" as a material is a non-count noun, the same as "bread"... but where predetermined units of bread are counted as "loaves," the equivalent units of cake are counted as "cakes." This means that the cake/cakes divide can be a bit wobbly, and in many situations either is acceptable.
English speakers (there is no distinction here between American, British, Canadian, etc. English) do say "cakes," but "The boy only eats chocolate cakes" is a pretty unusual sentence. This would mean "the only thing the boy eats are whole chocolate cakes" (i.e. he does not eat pieces of chocolate cake, he does not eat halves of chocolate cakes, but only entire cakes).
Presumably, the Hebrew sentence actually means "the only kind of food that the boy eats is chocolate cake."
To me, the difference between the singular and plural, in English, is 'The boy only eats chocolate cake' means the boy eats only chocolate cake and nothing else, whereas 'The boy eats only chocolate cakes' infers the boy likely eats many other things, perhaps he insists on unseasoned steamed kale at every meal, but when he does decide to eat cake, the only kind he eats is chocolate.
We can argue over the grammatical correctness of either cake or cakes, but the whole point of this course is: are you able to see the fine details in a sentence, and translate it properly, whether or not the sentence makes sense. My only question on this hebrew sentence is: the word "only" is used as a adverb in the translation :"only eats", so do adverbs in Hebrew work the same way as adjectives in that they follow the word that they modify? I suppose if you wanted to say "eats only chocolate cakes" then "only" would be modifying the the subject. So in that case would that then cause the רק to follow עוגות שקולד?
I agree, saying 'cakes' is wrong here. Just as you wouldn't normally say he eats only 'breads'. Cake here is a substance, a kind of food. A case where 'cakes' is appropriate is to refer to a number of whole cakes (not pieces of cake). Eg 'he ate four loaves of bread and seven cakes!'